The availability of a wind turbine is a measure of its ability to produce power when both the wind turbine and the grid are healthy and while ambient conditions are suitable, e.g. when wind speed is above the cut-in limit and below the cut-out limit. Availability is expressed as a percentage of uptime per turbine per year, for example 99% availability represents almost four days of lost production per year.
For cost-effective energy production, it is necessary to maximise the availability of wind turbines by minimising downtime. Wind turbines are complicated power plants and include various safety systems designed to protect the wind turbine from damage during adverse operating conditions. For example, wind turbines are designed to cut-out above a certain wind speed to avoid excessive loads acting on the blades and the tower.
Wind turbines also have a number of sensors for monitoring the temperature of various components, for example the generator, the gear oil and the hydraulic oil of the pitch control system. If the temperature of one of these components exceeds a predetermined threshold, then the wind turbine may be designed to de-rate (i.e. reduce its power output), or shut down completely. De-rating the wind turbine reduces the demand on the wind turbine in terms of power and/or speed and has the effect of stabilising or reducing the temperature of the components.
Wind turbines include cooling systems for controlling the temperature of temperature-critical components. For example, the gear oil is cooled by circulating it through a water-cooled heat exchanger. Wind turbines also typically have fans inside the nacelle for cooling the water in the heat exchanger and for cooling other temperature-sensitive components inside the nacelle. The fans are activated when the ambient temperature exceeds a predetermined threshold or if the temperature of the gear oil exceeds a predetermined threshold. If the cooling effect of the fans is not sufficient to maintain the temperatures below a safe operating level, then the wind turbine may need to be de-rated or shut-down. Equally, the wind turbine may need to be de-rated or shut down if there is a failure in any of the cooling systems causing the temperature of the critical components to exceed a predetermined safe operating level.
Wind turbines also include heaters for increasing the temperature of temperature-sensitive components in cold conditions and for maintaining the temperature of those components at the required level for effective operation. If the temperature of the components falls below a threshold, then start-up may be prevented and/or this may prevent the wind turbine from operating at its maximum power output level.
In order to maximise the availability of a wind turbine, the control systems of the wind turbine are generally designed to de-rate the wind turbine in response to an alarm event in preference to shutting the wind turbine down. An example of a power de-rate scenario of a wind turbine is described below with reference to FIG. 1. Specifically, the example shows how the turbine power is modulated as a function of the temperature of the gear oil in order to avoid a complete shut down of the turbine when the gear oil temperature becomes too high.
FIG. 1 is a plot of both the gear oil temperature TO and the power reference P versus time. Referring to FIG. 1, from time t0 to time t2, the wind turbine is operating normally. The turbine power P is at a nominal power level PN and the gear oil temperature TO is at a safe operating level. At time t1, there is a failure of the gear oil cooling system, which causes the gear oil temperature TO to increase steadily. At time t2, the gear oil temperature TO increases above a threshold safety level Tstart_d, triggering an alarm event. The threshold safety temperature level Tstart_d is slightly below a maximum gear oil temperature TM above which safe operation cannot be guaranteed. The alarm event signals to the wind turbine controller to de-rate the wind turbine, i.e. to operate at a reduced power output.
There is a correlation between power output and rotational speed and gear oil temperature, i.e. gear oil temperature TO is proportional to power output and to rotational speed, when all other factors are kept constant. Once the alarm event is triggered, the wind turbine is operated at a de-rated power that maintains the gear oil temperature TO at a reference level TR. At time t3, the cooling system is back online and working normally. From time t3 to time t4 the power output P is able to increase whilst the cooling system maintains the gear oil at the reference temperature TR. At time t4 the power output P has returned to the nominal power level PN and the gear oil temperature TO falls steadily. At time t5, the gear oil temperature TO falls below a threshold temperature Tstop_d at which point the wind turbine stops de-rated operation and returns to normal operation.
De-rating the wind turbine as described above in the event of an alarm is preferable to a total shut down because it allows the wind turbine to continue to operate once an alarm event is triggered, albeit at a reduced power level, thereby increasing the availability of the wind turbine.
If one of the temperature sensors or other sensors of the control system should fail, then incorrect data may be passed back to the controller, and the controller may take action to shut down the wind turbine unnecessarily due to an alarm event being triggered on the basis of spurious sensor data.
To avoid this problem, the applicant's co-pending PCT application WO2012/025121 describes a system for controlling a wind turbine on the basis of a theoretical state of the wind turbine if a sensor is determined to be faulty. For example, the control system is able to estimate the temperature of critical components such as the gear oil on the basis of other operating parameters, for example rotor speed, ambient temperature etc. If the sensor data is found to disagree significantly with the estimated temperature, then the sensor is deemed to be faulty, and the wind turbine is controlled on the basis of the estimated temperature values. This control method reduces the likelihood of the wind turbine being shut down or operated incorrectly if a sensor fails and hence increases the availability of the wind turbine.
It is desirable for wind turbines to be able to increase their yield (i.e. power output) wherever possible, for example to compensate for another wind turbine in the wind farm being shut down or if the grid operator demands a power boost. However, this may create a problem when the wind turbine is already operating close to its maximum temperature limits (e.g. in high ambient temperatures, and at high power outputs) because it may trigger an alarm event leading to the wind turbine being de-rated or shut down completely. As already described above, shutting down the wind turbine is undesirable at any time, but it is especially undesirable for a de-rate or shutdown to occur at the time when a power boost is required.
Against this background, the present invention aims to increase the availability of a wind turbine during varying operating conditions. The present invention also aims to provide a control strategy capable of responding to a power boost demand, particularly when the wind turbine is already operating close to its temperature limits.